This invention relates to control devices used in conjunction with heating pads when heating barrels and tanks, particularly those that are pressurised and which may contain hazardous materials.
In industry and life in general there is frequently found the necessity for heating the contents of a tank that is located in a place subject to the ravages of the outdoors and the associated elements. A practical method to warm the contents is by utilising an electric heater strapped to the tank. There are a number of such systems available, some of which are controlled by temperature sensing elements and some of which have no controls and they do heat tanks as required.
These systems in general include a heating element, a means by which to attach it to the tank and a power supply cable. In certain applications it is imperative that a limited amount of heat be applied to the tank so as to prevent overheating or over-pressurisation of the contents of the tank. To ensure that the tank contents do not go above a pre-determined safe set-point some systems limit the amount of power of the heater while other systems utilise a fixed point, temperature limiting thermostat as a safety cut-out while other systems use a combination of the two. There follows a list of problems associated with these types of systems when utilised for heating liquid pressurised gas (LPG) in tanks or cylinders.
LPG is used as a fuel over a broad range of temperatures in many areas of the world ranging from the arctic to the tropics. The mix of LPG is particular to each region and depends on the local requirements and each mix has its own heating requirements.
LPG is stored in pressurised tanks in the form of liquid and gas. The liquid portion being heavier fills the lower portion of the tank cavity and the gas being lighter fills the upper portion of the tank cavity. When the need arises, gas is withdrawn from the upper region of the tank and obeys the laws of chemical thermodynamics. As the gas is withdrawn the LPG temperature drops and the pressure within the tank drops thus making it difficult to obtain sufficient quantities of gas from the tank. By applying heat to the liquid contents of the tank the pressure within the tank is increased and greater quantities of gas may be withdrawn from the tank. If too much heat is applied to the tank the pressure could increase to the point where a safety release valve would be activated, releasing explosive gas into the atmosphere thus creating a dangerous situation. It is therefore desirable to heat the tank in a controlled fashion.
In the case of a heater that relies on a thermostat for safety there are three possible common configurations for the thermostat. In the first configuration the thermostat senses and limits the temperature of the heater. In the second configuration the thermostat senses and limits the temperature of the tank. In the third configuration the thermostat senses the ambient air temperature and in combination with a lower power of heater, ensures that the tank contents will not over-heat or become over-pressurised. In the case of the heater that has no thermostat the heating capacity is reduced so as not to overheat or over-pressurise the tank.
The problem with the systems that utilise the thermostats is that they are limited to pre-set operating temperatures which are not functional over a broad range of ambient temperatures. The problem with the systems that have no thermostats is that they have lower heating capacities and thus do not provide sufficient heating capacities in the warmer climates.